1. Field of the Invention
The present invention relates to a vibration welding method and a vibration welding apparatus for welding a resin base and a resin workpiece to each other as layers by clamping, pressing, and vibrating them with a pair of vibratory members which are vibrated relatively to each other.
2. Description of the Related Art
It has heretofore been customary in the art to join a resin component to another resin component by vibration welding. The vibration welding process requires no adhesive and is not followed by a subsequent process such as a heat drying process. The vibration welding process is advantageously used to join interior finishing members for automobiles, for example. Since importance is attached to the appearance of interior finishing members for automobiles, it is desirable to prevent the interior finishing members from being deformed by excessive vibration welding and also from being scratched by vibrations.
Japanese Laid-Open Patent Publication No. 2000-043567 discloses a bearing jig for a vibration welding product, having resilient members for absorbing partial thickness differences in superposed portions for thereby keeping the pressure of a vibratory jig at a constant level.
For welding members molded of one type of resin material, appropriate conditions may be set depending on the material and shape of the members to be welded thereby to achieve necessary appearance quality and joining strength.
Japanese Laid-Open Patent Publication No. 2001-232686 discloses a vibration welding jig having a urethane resin layer on its surface for contacting a product for thereby maintaining the appearance of the product. Specifically, the publication reveals the vibration welding of an air-conditioning duct member to an instrument panel as an interior finishing member of an automobile. The urethane resin layer on the vibration welding jig is provided as a protective member for the product surface, and has a uniform small thickness with no special consideration given thereto.
According to the related art disclosed in the latter publication, the duct member is supported on a lower jig which is shaped complementarily to the duct member, and the instrument panel is supported on an upper jig which is shaped complementarily to the instrument panel. The lower jig and the upper jig support the duct member and the instrument panel, respectively, over their entire surfaces.
A surface which supports the instrument panel is supported on the upper jig by the protective member, and is clamped together with the instrument panel and the duct member between the upper jig and the lower jig.
The upper jig is displaced toward the lower jig, and horizontally displaced back and forth relatively to the lower jig. The instrument panel and the duct member are pressed and vibrated by the upper jig and the lower jig.
The surfaces of the instrument panel and the duct member are held in frictional contact with each other and melted by frictional heat. Finally, the melted surfaces of the instrument panel and the duct member are solidified, joining the instrument panel and the duct member to each other.
The storage box of a front passenger SRS (Supplemental Restraint System) air bag, a duct of an air conditioner, etc. are joined to the reverse side of the instrument panel of the automobile by vibration welding. The storage box and the duct are made of different materials because of their required specifications. For example, the storage box is made of TPO (thermoplastic olefin) which is pliable and has a relatively low melting point, because it functions as a hinge for opening its front side at the time the air bag is inflated, and the duct is made of PP (polypropylene) having a relatively high melting point.
Heretofore, these different workpieces are joined to the instrument panel in different processes by vibration welding. If they are joined to the instrument panel in one process under the same pressure and vibrating conditions, then the material having the low melting point tends to be melted more than the material having the high melting point, and hence the materials are welded irregularity. Either one of the workpieces is liable to have an insufficient level of mechanical strength. Also, at least one of the workpieces is likely to be melted to an excessive depth, tending to deform the designed front surface of the instrument panel where the members are welded. Any deformation of the designed front surface of the instrument panel is not preferable because it is highly important for the designed surface to have a pleasing appearance.
When the storage box and the duct are joined to the instrument panel in different processes by vibration welding under different pressures and different vibrating conditions, they are melted to respective appropriate depths for respective appropriate joining strengths, and the designed surface of the instrument panel is not deformed.
However, the different vibration welding processes are time-consuming and inefficient because they need respective processes for loading and unloading workpieces into and out of the vibration welding apparatus and also other preparatory processes.
Either one of the workpieces tends to be deformed when they are welded under the same pressure and vibrating conditions because they have different shapes.
The instrument panel has a complex curved surface including different locations held against supports such as welding jigs at different tilt angles. Since the instrument panel is vibrated while being held under a uniform pressure in the vibration welding process according to the related art, the instrument panel is caused to move in directions other than the desired vibrational direction because of the different tilt angles. As a result, the instrument panel is not uniformly welded to the other members. The weld portions of the instrument panel may have their welding quality lowered, and the vibration welding apparatus may have its service life shortened due to undue forces applied thereto.
To solve the above problems, it may be proposed to use different vibration welding apparatus each for welding locations having the same tilt angle. However, using the different vibration welding apparatus is not economical and efficient because the overall cost of the vibration welding apparatus is high and the number of manufacturing steps involved is large.
In addition, the duct has a plurality of weld portions, and it is not efficient to weld those portions in separate processes.
The instrument panel has a complex curved surface and a considerably wide area. For supporting the instrument panel over its entire surface, a bearing member for supporting the instrument panel needs to be of a complex shape complementary to the instrument panel, and the amount of protective material needed is large. It is desirable to find a simple way of supporting the resin base which is complex in shape and wide in area, and also to reduce the amount of protective material used.
It may be effective to insert spacers in portions of either the lower jig or the upper jig for supporting the workpiece having the high melting point to prevent the workpiece from being welded irregularly. However, since the lower jig and the upper jig support the duct and the instrument panel over their entire surfaces, the thickness of the spacers has to be established in view of manufacturing errors of the dimensions of the instrument panel and the duct in order to allow the spacers to be inserted in those portions. Accordingly, a lot of trial and error efforts are required to design the spacers.
If a spacer with an excessively large thickness is inserted into a jig portion, then the pressure applied to the jig portion is excessively high. In that jig portion, though the welding strength of the instrument panel and the workpiece having the high melting point is large, the visible designed surface of the instrument panel tends to be deformed and suffers damage to its appearance.
When members made of materials having different melting points are joined to each other in different processes by vibration welding, the number of manufacturing steps is increased, and a long period of time is required until a final product is completed.